This invention relates to a RAKE reception method of receiving a spread spectrum signal in mobile communication.
In recent years, it is attempted to apply a spread spectrum communication system to mobile communication. This is because the spread spectrum communication system is highly tolerant to interference or jamming so as to achieve an excellent reception characteristic even in a multipath environment in which a transmitted signal is reflected and refracted by a variety of terrains or buildings to be received via different propagation paths (hereinafter called multipath propagation paths). Since the spread spectrum communication system uses a high spread frequency, it is possible to distinguish the multipath propagation paths from one another. This allows an application of a RAKE reception system. The RAKE reception system is a technique which provides a path diversity effect. Specifically, the RAKE reception system comprises the steps of separating the multipath propagation paths from one another, estimating propagation path characteristics of the individual paths, compensating reception signals of the individual paths with the propagation path characteristics to produce compensated reception signals, and coherently combining the compensated reception signals. In the RAKE reception system, a portion called a "finger" carries out the estimation of the propagation path characteristic of each individual path and the compensation therefor. For this purpose, the finger comprises a weighting factor estimating portion and a weighting circuit. The finger also includes a correlator for despreading a spread spectrum code (see Reference 1: Andrew J. Viterbi, CDMA/Principles of Spread Spectrum Communication, Addison-Wesley Publishing Company, page 89, 1995).
In case where propagation path delays in the individual multipath propagation paths can be separated from one another by the spread frequency, the RAKE reception system having the path diversity effect is extremely effective. However, if the difference in propagation distance among the multipath propagation paths is small such as in an urban area, path separation may be impossible in dependence upon the spread frequency. In this event, flat fading appears in the reception signals of the individual paths. In order to avoid the deterioration in reception characteristic due to the flat fading, it is effective to utilize a space diversity effect obtained by a plurality of antennas arranged apart from one another. Taking the above into consideration, it is proposed to combine the path diversity effect of the RAKE reception system and the space diversity effect by the plurality of antennas.
For example, Reference 2 (JP-A 8-237171) discloses a method of arranging a plurality of antennas as antenna branches, measuring RAKE reception levels of the individual antenna branches, and selecting a particular one of the antenna branches which has a highest reception level. Instead of switching among the antenna branches, Reference 3 (Higashi et al, "Path/Space Joint Diversity Performance of Coherent Detection RAKE with Interpolation on DS-CDMA", Proceedings of IEICE General Conference, B-410, 1995) and Reference 4 (Karasawa et al, "A Space-Path Hybrid Diversity Scheme for Base-Station Reception in CDMA Mobile Radio Communication Systems", Technical Report of IEICE, SAT93-12, pages 41-47, 1993) propose a system in which all multipath components received by the individual antenna branches are coherently combined so as to achieve a hybrid effect of the path diversity and the space diversity. In Reference 5 (Andoh et al, "Performance of RAKE and Space Diversity using Multi-pilot-block Channel Estimation for DS-CDMA", Proceedings of IEICE General Conference, B-5-13, 1997), the hybrid effect of the space diversity and the path diversity is confirmed by simulation tests.
Comparing the above-mentioned prior art techniques, the technique of coherently combining all multipath components received by the individual antenna branches (References 3, 4, and 5) is superior in reception characteristic over the technique of switching among the antenna branches (Reference 2).
In the above-mentioned prior art techniques, each antenna branch is fixedly connected to a set of the fingers. This structure is disadvantageous in the following respects. In the urban area with many tall buildings, there are an increased number of multipath propagation paths. In case of mobile communication, new propagation paths appear and old ones disappear very frequently so that the number of the propagation paths varies from time to time. Under the influence of shadowing, the reception characteristic of each individual antenna branch varies in a short time. In order to achieve an optimum reception structure, it is not desirable to permanently fix the connection between the antenna branch and the set of the fingers.
FIGS. 1 and 2 show the RAKE reception systems described in References 2 and 3, respectively. In either system, a preselected number of fingers are fixedly connected to each individual antenna branch. Each of the fingers has a correlator, weighting circuit, and a weighting factor estimating portion. In FIG. 1, RAKE combination is performed in a RAKE combiner for each individual antenna branch to produce a combined reception level. As a result of comparison, a particular antenna branch having a high reception level is selected in a switch SW. In FIG. 2, RAKE combination is performed in a single RAKE combiner to collectively combine all finger outputs of the both antenna branches. As far as a set of the fingers are fixedly assigned to each individual antenna branch, a total number of the fingers required in either system is equal to an integral multiple of the number of the branches. This results in an increase in size of the receiver.
In the prior art techniques, the branches or the paths selected for coherent combination are instantaneously switched upon reception of a small number of symbols. Thus, instantaneous variation of a reception signal received by each individual branch or path is excessively followed. Since the number of multipath propagation paths is great in the urban area as described above, the ratio of signal power to noise power (Eb/No) or the ratio of signal power to interference power (Eb/Io) for each path is small. Thus, other components than a desired signal component occupy a considerable part. Even if path selection is carried out in correspondence to a particular reception signal instantaneously exhibiting high correlation, it is not assured that a resultant reception characteristic is improved in average. In particular, the RAKE combination is effective provided that (1) propagation characteristics of the individual paths are statistically independent from one another and (2) each finger processes a proper reception signal.
In the spread spectrum communication system, when the Eb/No ratio or the Eb/Io ratio for each path is decreased at a particular station under the influence of the interference from other stations or the multipath propagation paths of the particular station itself, pseudo peak correlation appears. If the paths selected for RAKE combination are determined with reference to the pseudo correlation peaks, the resultant reception characteristic is undesiredely deteriorated. Therefore, the fingers selected for RAKE combination must be determined in dependence upon the characteristics of the reception signals. In particular, the paths selected for RAKE combination must be determined so that the average reception characteristic is improved. Since the multipath variation or the variation of the reception signal in each individual antenna branch is in situ probabilistic, high correlation instantaneously obtained does not assure that a path or a branch which is proper in average is caught. In order to avoid any improper path from being caught due to instantaneous variation, it is necessary to incorporate a mechanism for protecting synchronization.